Renal artery stenosis (RAS) is a potential remediable cause of hypertension. Without revascularization, there is continued progression of the renal artery stenosis; the eventual outcome is often end-stage renal disease, with its associated high morbidity and mortality. Although contrast angiography is considered the "gold standard" for diagnosis, it is invasive and expensive, and does not provide information about the functional significance of the lesion. Captopril renography is commonly used to screen for functional renal artery stenosis, but its accuracy is reduced with azotemia or bilateral disease, and it does not provide anatomical information. An improved noninvasive test for renal artery stenosis could be of enormous value. Recent advances in the field of computed tomographic angiography (CTA) and magnetic resonance angiography (MRA) have opened up the possibility of noninvasive angiography of the renal arteries. Each modality has shown promise in initial human studies, but significant technical limitations remain. The investigators propose technical development and clinical testing of both modalities using a combination of phantom studies, an animal model, and studies of healthy volunteers and patients. The specific aims of this proposal are: 1. To optimize the imaging parameters for 2D STAR, and breath-hold contrast-enhanced 3D and multisection 2D MRA in a pulsatile flow phantom, and in normal volunteers. Additionally, both MRA and CTA techniques will be optimized in a porcine model of renal artery stenosis. 2. To test the hypothesis that MRA and CTA accurately depicts normal renal anatomy, including accessory vessels. 3. To determine the sensitivity and specificity of MRA and CTA in patients with suspected renal artery stenosis. 4. To test the hypothesis that the use of MRA and/or CTA is cost-effective in patients with suspected renal artery stenosis.